What Is A Timing Belt?

A timing belt is a toothed belt made of reinforced rubber that rides on sprocket wheels. Driven by the engine's crankshaft pulley, a timing belt provides rotational force for camshafts that open and close intake and exhaust valves at precisely the right time for smooth combustion. Because the valves must be synchronized with the operation of the engine's intake, compression, combustion, and exhaust strokes, the timing belt ensures that it all happens. In this article, we'll discuss how timing belts wear out, and why it's critical to replace them when the time comes due.

Timing belts, to a large degree, began replacing
timing chains (which serve the same purpose) on smaller displacement engines during the 1970s and 80s because they weighed less and weren't as costly to produce. However, vehicles with larger displacement engines have continued to use timing chains because greater strength is needed to spin more massive camshafts that operate a large number of valves. For comparison, a timing chain lasts much longer than a belt - sometimes for the lifetime of the vehicle. However, for the purposes of this article, we'll focus on timing belts.

Depending on engine design, a timing belt may also spin other pulley wheels that drive a coolant pump, oil pump, or fuel pump. So it's easy to see what an important role a timing belt plays. Without it, you simply cannot have a functioning engine.

A closeup look at the grooved teeth of a timing belt.

During the first two or three decades that timing belts existed, vehicle manufacturers typically recommended replacing them as a maintenance item between 45,000-60,000 miles. Some cars had recommendations as early as every 30,000 miles. As rubber compounds have improved and become more durable, replacement intervals have climbed to 100,000 miles or more.

Why Replacing A Timing Belt Is So Important

Like any item made of rubber, a timing belt will eventually dry up, harden, crack, and break apart. If a timing belt should break, your engine will stop running immediately because intake and exhaust valves will grind to a halt - causing compression in the cylinders to be completely lost. But when valves stop moving in synch with the rest of the engine, you may have bigger problems than a stalled engine that won't restart.

Damage to valves and the cylinder head(s) that house them can result if your engine has what's known as an "interference" design. On an interference motor, both intake and exhaust valves extend further into the cylinder area during the time when they're opened. This is an engineering decision that is done for performance or emission purposes. Normally, proper timing means pistons are never trying to occupy the same space as valves. But if valves come to a halt too far into the cylinder head, the pistons will smash into them before they too come to a stop.

On this 4-cylinder engine, pistons and valves collided and were destroyed when the timing belt broke. A complete engine rebuild was required.

Permanent damage to the valves will result, along with bearings and other related components. In some cases, pistons and cylinder head(s) can become damaged also - with rods and valves pushing their way through the metal. Cases exist where piston connecting rods have punched through oil pans after a crankshaft broke because of violent shock.

Engines with a "non-interference" design will not suffer piston-and-valve collision damage, but you'll still be stranded. If you're not sure about which type of setup your engine has, a call to the vehicle manufacturer should provide an answer.

Signs Of Wear

Because timing belts are safely tucked behind a cover plate, they're not directly exposed to detrimental elements from within the engine compartment. Unfortunately, this fact also makes performing a casual eyeball inspection impossible. And because timing belts have teeth on them, worn ones won't slip enough to create the traditional loud, telltale squealing noise that worn serpentine belts typically make. However, a worn timing belt may create a faint ticking noise from behind the engine cover - but it's possible such a noise could be caused by other unrelated issues.

In order to see the condition of a timing belt, one must remove the cover plate which essentially forms the very front of the engine. More specifically, various accessory drive belts and other external components bolted to the front of the engine must come off first.

Without being able to see, hear, or feel signs of wear, it's simply a must to replace any original timing belt at the manufacturer's recommended mileage point. An exception must be made for vehicles driven very infrequently. Rubber deteriorates over time, so there certainly will be cases when timing belt replacement is warranted after X number of years, before the stated mileage interval is reached. Many manufacturers will cover this by stating something like: "Replace timing belt after 100,000 miles or 10 years whichever comes first".

If you've purchased a used vehicle and don't have service records that document any history of timing belt replacement, the only safe and practical thing to do is presume the belt has not been changed on schedule. As we mentioned, the labor to inspect a timing belt is basically the same as replacing it. So go ahead and figure on replacing it - unless an inspection reveals a belt that's obviously brand new.

Signs Of Trouble - Without Having To Perform A Visible Belt Inspection

If you see oil leaking from the timing belt cover at the front of the engine, it indicates a front engine seal is leaking, or a front cover which is supposed to seal in oil has a leaky gasket. Regardless of the reason, engine oil is extremely detrimental to rubber - causing it to soften and come apart. If the timing belt has become contaminated with oil, replace it and address the source of the leak immediately.

Like all rubber belts, timing belts can become stretched a little bit as they age. This can result in the belt occasionally slipping a tooth on the pulley wheel. The engine will continue to run, but valve timing may be off just enough to create a misfire situation. As rpms increase, the engine may run rougher and churn out more smoke as a result. On newer cars, this can cause the Check Engine light to illuminate.

Signs of Trouble During A Visible Belt Inspection

Beware of cracks on your timing belt.

As we mentioned before, if you get the timing cover off and find oil on the belt, the leak needs to be addressed immediately and a new belt should be installed. Cracks on the belt or missing teeth are the most common visual indicators it needs replacement.

Old timing belts past their prime tend to suffer from dried rubber, which can cause teeth to break off.

It's important to note that rubber material loss can occur evenly along all surfaces of the teeth during normal wear. Gradually, the rubber that has worn off tends to collect in between the teeth, dulling the sharp edges and causing vibrations. A belt that's lost rubber will not be as tightly held by the tensioner, so it's more likely to slip a tooth out of position.

The belt on the top has suffered significant material loss on the teeth compared to a new one below.

Opportune Times To Replace A Timing Belt

If you're replacing a water pump or doing anything else that requires removing the timing belt cover at the front of the engine, there's very little extra labor to slip a new belt in place once you get in there. We say this because removing the timing belt cover plate can be the majority of the labor due to serpentine accessory drive belts and other external components which must be taken off first. Conversely, if you're replacing your timing belt at the recommended interval, there's minimal labor to replace the water pump because you've already done all the work of getting to it in the first place.

And if replacing the water pump is the prime concern, re-installing the old belt isn't a great idea. It may have stretched just enough to make re-setting the camshaft timing real challenge.

Pulley Wheels & Tensioner

We also strongly encourage replacing the tensioner for the timing belt while you've got access to it. Additionally, check the condition of any idler pulley wheels put in place to help determine the belt's path. All of these items have bearings in them that wear down gradually over time, and they're usually ready for replacement when the belt wears out. A faulty tensioner will create noise and vibration that can be heard if listening closely to the front of the engine.

Installing a new timing belt tensioner when you change the belt makes good sense.

There are definite signs that a timing belt tensioner or idler pulley wheel is worn and can no longer maintain proper alignment. A belt with frayed edges indicates that wobbling of the pulleys is causing the belt to ride slightly askew. This wobbling and misalignment can occur because mounting bolts that secure these items in place have loosened, or because internal bearings have worn down to the point where they can no longer keep the pulley supported in a flush and level position.

Belts that are worn in this fashion are a result of pulley wheels or a tensioner that has become misaligned due to wear.

Remove the timing belt and see if the tensioner and pulleys are tight, or if they have side-to-side play and can be wiggled excessively. Rotate the pulley wheels to see if they spin easily, or if roughness can be felt as they turn. Anything that isn't right should be replaced.

If you plan on replacing both your belt and tensioner at the same time, we recommend the Preferred Components
Timing Belt Set for a number of vehicles, and the FEBI Timing Belt Kit for Audi and Volkswagen models. We've also got a
number of individual pulley wheels as well.

When you consider the cost of an engine rebuild, the price of a new timing belt, pulley wheels, and tensioner is literally pennies by comparison. Skimping on these maintenance items is skirting disaster - so make a smart financial decision and replace them on schedule!

Michael Grayen is an SAE certified mechanic and Technical Writer at CARiD. A resident expert and professional, Mike is passionate about all things automotive and loves to share his knowledge. When not at work you can probably find him at a cruise night or working on a muscle car. His articles and write-ups cover everything from routine maintenance to performance and styling modifications. Follow him on G+, Facebook and Twitter.

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